Aluminium salt adjuvants are currently the most widely used adjuvants for human and veterinary vaccines. Aluminium adjuvant compounds include aluminium salts such as aluminium phosphate (AlPO4) and aluminium hydroxide (Al(OH)3) which are generically referred to in the field of vaccine adjuvants as “alum”. To provide adequate immunogenicity, it is thought that antigens must be adsorbed onto the surface of the adjuvant. It is believed that alum adjuvants act as an immune system stimulus as well as providing a depot of antigen at the site of administration (e.g. by injection) thereby providing a gradual and continuous release of antigen to stimulate antibody production. Aluminium adjuvants in their natural form are commonly known as gels, which are particulate suspensions in aqueous media.
The storage and transportation of alum-adjuvanted vaccines is problematic. Freeze-drying (lyophilisation) is a process frequently used to improve long-term stability of various protein preparations. Nevertheless, commercial vaccine compositions containing aluminium salt adjuvants cannot be freeze-dried without causing damage to the adjuvant structure. Freeze-drying causes the collapse of the gel structure of the adjuvant resulting in aggregation and precipitation of the adjuvant salt on resuspension in water. The effect is to significantly reduce the immunogenicity of the vaccine.
WO 01/93829 describes a method of preparing an adjuvanted vaccine comprising spray-drying or spray freeze-drying an aqueous solution comprising:    (a) from 0.1 to 0.95% by weight of an aluminium salt or calcium salt adjuvant having an antigen adsorbed therein;    (b) from 0.5 to 6% by weight of a saccharide;    (c) from 0.1 to 2% by weight of an amino acid or salt thereof; and    (d) from 0.02 to 1% by weight of a colloidal substance.
WO 2008/118691 describes a method of preparing an immunologically-active adjuvant-bound dried vaccine composition comprising (a) combining at least one aluminium-salt adjuvant, at least one buffer system, at least one glass-forming agent and at least one antigen to create a liquid vaccine formulation; (b) freezing the liquid vaccine formulation to create a frozen vaccine formulation; and (c) lyophilizing the frozen vaccine formulation to create a dried vaccine composition. The glass-forming agent is preferably trehalose.